Method for centering and dimensioning an image on a cathode-ray tube

ABSTRACT

A method for centering and dimensioning an image on a cathode ray tube receiving display signals supplied by a display calculator includes measuring durations of vertical black edges of the image, and modifying adjustment values for a horizontal centering of the image to obtain equal lateral vertical edges. The method further includes measuring the durations of the vertical black edges of the image to calculate adjustment values for a horizontal dimension of the image to cause the vertical black edges to disappear. Durations of the horizontal black edges of the image are measured to calculate adjustment values of a vertical dimension of the image, and to calculate adjustment values of a vertical centering of the image to cause the horizontal black edges to disappear for centering the image vertically. The adjustment values obtained are recorded in a memory of the display calculator.

The present invention relates to cathode ray tubes used for displayingimages on television (TV) sets and personal computers (PC). Moreparticularly, it relates to a method of automatically centering anddimensioning a displayed image.

A cathode ray tube for displaying images on a screen formed by itsfaceplate generally comprises electronic circuits which control thescanning of the screen by an electron beam so as to activate or not theluminescence of screen pixels and thus produce the desired image.

These electronic scanning circuits are driven via a display controllerthrough electrical signals which can have different sources, such ascomputer signals, signals from laser disks or game consoles. Because oftheir diversity, a same setting for the size and position of the imagecannot be suitable for all sources and can result in a bad centering ofthe image on screen and a distorted image. These defects can moreoverexist for a cathode ray tube from the factory in the case where thesettings are not properly adjusted.

This can account for the presence of black stripes on the vertical orhorizontal edges of the image, a horizontal or vertical shift of theimage, or an image distortion in the horizontal or vertical direction.

In the state of the art, these faults are corrected manually by the userthrough control buttons which bring up adjustment menus and sub-menus.Such an operating procedure is neither fast nor simple, notably owing tothe fact that there are only few control buttons, which requires theuser to employ a same button for several different functions.

This is all the more inconvenient as these adjustments must be made as afunction of the screen's operating mode, for example to pass from onevideo mode to another or from a classical 640×480 pixel screen to ahigher definition 1280×1024 pixel screen.

Accordingly, when changing from one video mode to another, the displaycalculator analyzes the new horizontal and vertical synchronizationsignals, calculates their frequencies and carries out the necessaryadjustments to display a new image in the new mode. However, the imageobtained is never perfectly adapted to the screen size, and consequentlysuffers from faults as regards centering and dimensioning or sizementioned above.

Therefore, in the state of the art, the user must activate the controlbuttons provided for that purpose until the desired image is obtained,and the adjustments made are written into a memory of the displaycalculator, not only for the current session, but also for latersessions with the same display mode.

However, despite this memory storage of the settings, the user oftenneeds to readjust the latter during a subsequent use of the same displaymode, and all the more so as the settings of the mode entered in memoryare not always adapted to all software which use that mode.

An object of the present invention is thus to implement a process forautomatically centering and dimensioning an image on the screen of acathode ray tube.

The invention relates to a method of centering and dimensioning an imageon a cathode ray tube whose display signals are supplied by a displaycalculator, the method being characterized in that it comprises thefollowing steps:

(a) measuring the durations of the vertical black edges of the image andmodifying step by step the adjustment (HPOS) for horizontal centering toobtain equal lateral vertical edges;

(b) measuring the durations of the vertical black edges of the image tocalculate the adjustment (HSIZE) of the horizontal dimension of theimage so as to cause the vertical black edges to disappear;

(c) measuring the durations of the horizontal black edges of the imageto calculate the adjustment of the vertical dimension of the image(VSIZE′) and the adjustment of the vertical centering of the image(VPOS′) so as to cause the horizontal black edges to disappear and tocenter the image vertically, and

(d) recording the adjustment values obtained (HPOS, HSIZE, VSIZE′ andVPOS′) in a memory of the display calculator.

Steps (a), (b) and (c) can be performed in any order because they areindependent of each other, but it is advisable to perform step (a)before step (b), given that the precision for the calculation of thesetting (HSIZE) for the horizontal dimension of the image depends on theperfect horizontal centering of the image.

Step (d) can come into play after each step (a), (b) or (c) to recordthe value of the setting obtained by the step having just been finished.

The method is implemented only if the image is sufficiently stable, thisbeing detected by checking that the positions of the vertical andhorizontal edges have fluctuations below a certain threshold. Thisstability is checked before each step (a), (b) or (c)

Other characteristics and advantages of the present invention shallbecome apparent from reading the following description of a preferredembodiment, in relation with appended drawings in which:

FIG. 1-A shows an image on a screen of a cathode ray tube which is notcentered and exhibits a black surround, and FIG. 1-B shows the sameimage after implementation of the process in accordance with theinvention;

FIG. 2 is a diagram showing the relations between the horizontalposition of the image on screen and the horizontal scanning signal foran image line;

FIG. 3 is a diagram analogous to that of FIG. 2, but showing therelations between the vertical position of the image on screen and thevertical scanning signal for a complete image;

FIG. 4 is a diagram showing the main steps of the process in accordancewith the invention;

FIGS. 5-A and 5-B show the steps in the horizontal image centeringalgorithm in accordance with the process of the invention, thisalgorithm being preceded by an algorithm for checking the imagestability;

FIG. 6 is a curve showing the variation of H_(AMPMIN) as a function ofthe horizontal scanning frequency for a given range of frequencies; and

FIG. 7 is a diagram showing the adjustment for vertical centering andvertical dimensioning.

FIG. 1-A shows the faceplate 10 of cathode ray tube 12, on the screen ofwhich appears an image 14 whose vertical edges 16 and horizontal edges18 are black (i.e. dark), so indicating that the image 14 is notcentered at the center of the screen and that it only occupies a part ofthe screen.

As indicated in the introductory portion above, the adjustments forcentering and dimensioning the image are at present made by the userthrough buttons 20 which bring down menus and sub-menus on the screen toguide the user in the adjustments.

These control buttons 20 are active for the adjustments via a displaycalculator which supplies the values of horizontal and vertical scanningsignals. This display calculator is capable of receiving the videosignals and analyzing them to output these scanning signals.

In accordance with the invention, a control button 22 (FIG. 1-B) isadded to implement the inventive process and obtain in a few seconds thecorrectly centered and dimensioned image of FIG. 1-B.

The process of the invention is based on the measurement of the length,in units of time, of vertical and horizontal black edges, thesemeasurements then serving for carrying out algorithmic operations andcalculations leading to a modification of the image centering and itsdimensions.

FIG. 2 shows the image 14 and the corresponding horizontal scanningsignal 30 as a function of time t for a line of the image, i.e. thecurrent I_(H) flowing in the horizontal deflection coil (yoke). Thefigure also shows the horizontal synchronization pulses 32 and 34(HFBACK) which determine the start and end points of a horizontalscanning signal, the start of horizontal scanning corresponding to thefalling edge of pulse 32 and the end corresponding to the rising edge ofpulse 34. The duration of the scanning return (flyback) is given by theduration of pulse 32 or 34.

When the image exhibits vertical black edge portions, this comes fromthe fact that signals of the Red, Green and Blue components at the startand end of horizontal scanning are all below a certain level. Themeasure of the time duration T1_(HAV) between the falling edge and thestart of the left of the image indicates the extent of the left verticalblack edge portion while a measure of T2_(HAV) between the end of theright of the image and the rising edge indicates the extent of the rightvertical black edge portion.

It then follows that if T1_(HAV)=T2_(HAV), then the image is centeredhorizontally, whereas it is not centered if T1_(HAV) is different fromT2_(HAV).

The process in accordance with the invention obtains horizontalcentering of the image by:

-   -   measuring T1_(HAV) and T2_(HAV) in a repetitive manner,    -   comparing T1_(HAV) and T2_(HAV) at each time,    -   displacing the image by one unit towards:        -   the right if T1_(HAV)<T2_(HAV),        -   the left if T1_(HAV)>T2_(HAV) until is obtained the equality            T1_(HAV)=T2_(HAV).

The measurement of T1_(HAV) and T2_(HAV) is performed by the displaycalculator using a device provided to that effect and known per se.

TI_(HAV) and T2_(HAV) do not allow to obtain the horizontal dimensioningof the image for making the vertical black edge portions disappear,since the time interval between two horizontal synchronization pulses 32and 34 is fixed, irrespective of the horizontal width of the image. Theprocess of the invention produces this horizontal dimensioning bymodifying the amplitude of the curve 30 in accordance a formula, asshall be described below.

FIG. 3 shows the image 14 and the corresponding vertical scanning signal40 as a function of time t for a complete image, i.e. the voltage V_(v)of the vertical deflection sawtooth signal for line-by-line verticalscreen scanning. The figure also shows the vertical synchronizationsignals 42 and 44 (VFBACK) which determine the start and end points of avertical scanning signal, the duration of the pulse determining theduration of the return period for the vertical scanning signal.

As in the case of horizontal line scanning, the time periods T1_(VAV)and T2_(VAV) respectively define the extents of the top black edgeportion and the bottom black edge portion of the image. However, thesetime periods cannot serve to center the image vertically because thetime interval between the top and bottom edges of the image and thecorresponding pulses 42 and 44 remain constant irrespective of thevertical position of the image.

Likewise, the time periods T1_(VAV) and T2_(VAV) cannot serve directlyfor vertically dimensioning the image because the time period of thevertical synchronisation pulses 42, 44 remains the same irrespective ofthe image height. The measurement of T1_(VAV) and T2_(VAV) is carriedout by the display calculator using the above-mentioned measuring devicefor measuring T1_(HAV) and T2_(HAV).

The process according to the invention provides the vertical centeringand the vertical dimensioning by modifying the amplitude of the curve 40in accordance with a formula as shall be described hereafter.

The diagram of FIG. 4 illustrates the main steps of the invention, whichcomprises the steps of:

(a) measuring T1_(HAV) and T2_(HAV) to calculate the adjustment HPOS toperform in order to obtain the horizontal centering of the image (box50),

(b) measuring T1_(HAV) and T2_(HAV) to calculate the adjustment toperform HSIZE in order to obtain the horizontal dimensioning of theimage (box 52),

(c) measuring T1_(VAV) and T2_(VAV) to calculate the adjustment toperform VPOS and VSIZE in order to obtain at the same time verticalcentering and the vertical dimensioning of the image (box 54), and

(d) recording the values HPOS, HSIZE, VPOS′ and VSIZE′ in a memory (box56) of the display calculator.

If an error arises during one or another of steps 50, 52 and 54, notablyin the case of image instability, the starting values are restored inthe memory (box 58). These errors can arise from an image which isunstable, which is shifting, which is too small to be adjusted, or forany other reason.

Note that steps (a), (b) and (c) can be performed in any order, but itappears logical to start with the simplest, which is the horizontalcentring step, owing to the fact that it stems directly from themeasurement of T1_(HAV) and T2_(HAV). Moreover, step (b) yields moreprecise results if it follows from step (a).

The diagram of FIGS. 5-A and 5-B shows in detail the operations to beperformed during step (a) for horizontal centring. However, the firstoperations 60, 62, 64, 66, 68 and 70 are repeated, wholly or in part, atthe start of each step (a), (b) or (c) to check that the image is stablewithin the established limits. These first operations comprise the stepsof:

-   -   pressing on button 22 (arrow 60) to trigger off the operations,    -   performing a first series of measurements to obtain a first set        of pairs of values T1_(HAV1) and T2_(HAV1), T1_(VAV1) and        T2_(VAV1) (box 62),    -   performing a second series of measurements to obtain a second        set of pairs of values T1_(HAV2) and T2_(HAV2), T1_(VAV2) and        T2_(VAV2) (box 64),    -   subtracting the second set of pairs of values from the values        from the first set (box 66) to obtain difference values DIFF in        terms of absolute values,    -   comparing the difference values DIFF with a threshold TMUDIFF        (lozenge 68),    -   stopping the operations if DIFF>TMUDIFF, for the image is then        considered to be unstable or shifting, or passing onto the next        operation (lozenge 70) in the opposite case.

Note that the series of measurements T1_(AV) and T2_(AV) which concernthe horizontal deflection are preferably only performed just before eachhorizontal adjustment (a) or (b) to determine the horizontal stabilityof the image.

Likewise, the series of measurements T1_(VAV) and T2_(VAV), whichconcern the vertical deflection, are only performed just before thevertical adjustments, preferably for centering and dimensioning todetermine the vertical image stability,

-   -   comparing T1_(HAV) and/or T2_(HAV) (lozenge 70) with a maximum        value MAX and stopping the operations if it is reached, for the        image is then considered to be too small and hence not        exploitable, or that the video signal is bad (lozenge 70); in        the case of a negative comparison, passing on to the next        operation, the first concerning the horizontal centering proper,        which comprises the steps of:        -   checking whether the negative comparison arrives for the            first time or not (lozenge 72), and        -   in the case of a positive check, passing on to the next            operation comprising the steps of:    -   comparing T1_(HAV) with T2_(HAV) (lozenge 74), and    -   stopping the horizontal centering operations in the case of an        inequality, for the image is already horizontally centered, and        passing on to step (b),    -   the image must be displaced to the right if T1_(HAV)<T2_(HAV),        such an event being memorized by a flag at the 0 state,    -   the image must be displaced to the left if T1_(HAV)>T2_(HAV),        such an event being memorized by the flag, but in this case at        the 1 state,    -   in the case of a negative check, or in the case where the image        must be displaced, passing on to the next operation.

The value 0 or 1 for the flag indicates the direction in which the imageis to be displaced, the displacement being effected in a stepwise mannerby incrementing or decrementing the centering adjustment value HPOS.

The following operations involve comparing T1_(HAV) with T2_(HAV) andmodifying the centering adjustment value HPOS in the direction indicatedby the value of the flag until detection of the equalityT1_(HAV)=T2_(HAV). These operations are presented in the diagram of FIG.5-B.

The first operation (box 80) consists in checking whether the flag is atlogic 1, indicating that the image is off-centered in the rightdirection and must be brought back to the left.

-   -   if the check is positive, the following operation consists in        checking whether T1_(HAV)>T2_(HAV) (lozenge 82), and there are        three possible responses:        -   (i) T1_(HAV)=T2_(HAV), in which case the image is centered            and the horizontal centering operations are stopped to pass            on to step (b),        -   (ii) T1_(HAV)>T2_(HAV), in which case the image is            off-centered in the right direction and must be displaced to            the left by decrementing the adjustment value HPOS by one            unit (box 86); moreover, a loop counter 90 is incremented by            one unit;        -   (iii) T1_(HAV)<T2_(HAV), in which case the image which was            off-centered in the right direction since the start of the            operations is now off-centered towards the left, which means            that the centering value HPOS has been exceeded by one unit.            This overshoot is corrected by incrementing the horizontal            adjustment value HPOS by one unit (box 94). With this            incrementation, the value of HPOS corresponds to the center            position, and the horizontal centering operations are            stopped to pass on to step (b).

If the flag is not at logic 1, i.e. the image is off-centered to theleft and must be brought back to the right, the following operation (box84) consists in checking whether T1_(HAV)<T2_(HAV), and there are threepossible solutions as in the previous case:

-   -   (i) T1_(HAV)=T2_(HAV), in which case the image is centered and        the horizontal centering operations are stopped to pass on to        step (b),    -   (ii) T1_(HAV)<T2_(HAV), in which case the image is off-centered        in the left direction and must be displaced to the right by        incrementing the adjustment value HPOS by one unit (box 88);        moreover, a loop counter 90 is incremented by one unit;    -   (iii) T1_(HAV)>T2_(HAV), in which case the image which was        off-centered in the left direction since the start of the        operations is now off-centered towards the right, which means        that the centering value HPOS has been exceeded by one unit.        This overshoot is corrected by decrementing the horizontal        adjustment value HPOS by one unit (box 96). With this        incrementation, the value of HPOS corresponds to the center        position and the horizontal centering operations are stopped to        pass on to step (b).

If the loop counter 90 is incremented, this means that the centeringvalue HPOS has not yet been obtained and that it is necessary startagain all the operations described above (new loop) starting from step62 consisting of measuring new values of T1_(HAV) and T2_(HAV)subsequent to the new value of HPOS.

However, this new loop is performed only if the number of loops has notexceeded a certain threshold BMAX. The operation consists in:

-   -   comparing (lozenge 92) the contents of the loop counter 90 with        BMAX,    -   stopping the operations if the centering has not been achieved        after a set number of shifts BMAX,    -   or starting a new loop if BMAX is not attained.

To set the horizontal dimension of the image such that it takes up theentire width of the screen, i.e. without vertical black edges, it isnecessary to change the amplitude setting for the current flowing in thehorizontal deflection coil, such an adjustment being represented by avalue HSIZE which can vary e.g. between 0 and 255. It is this valueHSIZE for obtaining a maximum image width which is calculated by themethod according to the invention, this value being dependent on manyparameters, and notably T1_(HAV) and T2_(HAV).

The formula which enables to calculate HSIZE is:

${HSIZE} = {\frac{{A_{Vopti}\left( {T^{\prime}/{Td}^{\;\prime}} \right)} - H_{AMPMIN}}{H_{AMPMAX} - H_{AMPMIN}} \times {HSIZE}_{MAX}}$

In which formula:

-   -   A_(Vopti) is the optimum amplitude of the current in the        horizontal deflection coil to obtain an image of optimum width;        this amplitude is measured for a type of cathode ray tube and in        a reference video mode,    -   HSIZE_(MAX) is the maximum value of HSIZE, e.g. 255 as indicated        above,    -   H_(AMPMAX) is the maximum variation of the current in the        horizontal deflection coil to obtain a maximum horizontal        deflection; this value varies as a function of the horizontal        scanning frequency f(fH) as described below;    -   H_(AMPMIN) is the minimum variation of the current in the        horizontal deflection coil to obtain a minimum horizontal        deflection; this value varies as a function of the horizontal        scanning frequency f(fH) as described below;    -   T′ is the total duration of a horizontal line, i.e. the duration        of the period T of the horizontal synchronization signal, from        which are subtracted the duration of the flyback pulse        T_(FLYBACK), in general three microseconds, and a duration of        safety margins, e.g. 0.6 microseconds, and        −Td′=T−(T1_(HAV) +T2_(HAV) +T _(FLYBACK))

i.e. the duration of the image on screen between these black edges.

This formula is established by supposing that the current varieslinearly, which is not the case, so that to take into account the factthat the curve is S shaped, the coefficient applied A_(Vopti) must bereplaced by(1.8T′−Td′)/2.8Td′,

which coefficient can change depending on the type of cathode ray tubeand its control device.

The values for H_(AMPMIN) and H_(AMPMAX) are determined by means of,curves as a function of the horizontal scanning frequency f(H), thisbeing effected for frequency ranges.

For instance, curve 100 of FIG. 6 shows the function of H_(AMPMIN)=f(fH)for a range of frequencies from 34 kHz to 41 kHz for the case of a givencathode ray tube. The abscissa x is graduated in kHz while the ordinateis graduated in H_(AMP) ×10 mA. There is thus obtained a straight linewhose equation is:Y=−5.22x+1265.1=ax+b.

This equation is different for another range of frequencies.

Coefficients a and b determined for each range of frequencies arerecorded in a memory so that they can be read in view of calculatingH_(AMPMIN) according to the horizontal scanning frequency.

H_(AMPMAX) is obtained in the same manner as for H_(AMPMIN).

As a result, if there are eight frequency ranges, there shall be sixteenpairs of coefficients (a,b) which define the sixteen variation curves,eight for H_(AMPMIN) and eight for H_(AMPMAX).

To achieve vertical centering and vertical dimensioning, the methodaccording to the invention consists in measuring the values T1_(VAV) andT2_(VAV) for the image which appears on the screen, and then firstcalculating VSIZE to obtain the vertical dimensioning and subsequentlyVPOS′ to obtain the vertical centering according to the followingformulae:VSIZE′=0.5[(3VSIZEMAX+2VSIZE)]·[(Td×T′)/(TD′×T)]−1.5(VSIZEMAX) andVPOS′=VPOS+(A−B)withA=[(2.25+1.5.(VSIZE/VSIZEMAX)]×[(0.5−T1/T).(VPOSMAX/0.6)]andB=[(2.25+1.5.(VSIZE′/VSIZEMAX)]×[0.5−T1′/T′).(VPOSMAX/0.6)]

To define the parameters of these formulae, reference shall be made toFIG. 7, which shows the sawtooth for vertical scanning 40, but inversedwith respect to that of FIG. 3. The abscissa shows the duration and theordinate shows the voltage V_(OUT). On this sawtooth is placed the image112 to be vertically centered and dimensioned and a reference imagewhich is appropriately vertically centered and dimensioned.

In FIG. 7, T₁, T₂ correspond in time periods respectively to the startand end of the reference image 114, while T1′ and T2′ correspond in timeperiods respectively to the start and end of the image 112 to becentered and dimensioned. The following relations are then established:

The duration Td of the reference image is given by Td=T2−T1, and theduration Td′ of the image to be centered is given by Td′=T2′−T1′.

Also, T1=T1_(VAV) and T2=T−T2_(VAV), T being the total duration of asawtooth. Similarly, T1′=T1′=_(VAV) and T2′=T′−T2′_(VAV).

The reference image 114 is obtained by a manual adjustment in areference video mode on a given type of cathode ray tube and the valuesT1_(VAV) and T2_(VAV) are measured and entered into a memory to be usedfor the automatic adjustments on that type of cathode ray tube. The sameapplies for the value VSIZE, which corresponds to that reference image,while VZIZEMAX is the maximum adjustment value, for example 256.

These elements allow to calculate the value VSIZE′ according to theabove formula, i.e. the adjustment value that will allow to obtain animage which is appropriately vertically dimensioned.

By knowing VSIZE′, it is possible to calculate VPOS′ according to theabove formula, which also uses the value VPOSMAX, which is the maximumadjustment for the vertical centering.

The invention has been described in its application to the adjustment ofa cathode ray tube by the user of a computer or a television set inwhich the cathode ray tube forms the display screen. The invention alsoapplies to the implementation of the process for adjusting thehorizontal and vertical deflection coils at the end of a cathode raytube production line.

Indeed, at the end of a cathode ray tube production line, the imagegenerated to test for the correct operation of the cathode ray tubeexhibits faults which an operator corrects in various ways. One of thefaults concerns a bad alignment between the image and screen centersand, to correct it, the operator first performs image centering anddimensioning adjustments using the buttons 20 (FIG. 1-A) and then theadjustments in the electronic and magnetic circuits (deflection coils)to displace the image center and make it coincide with the center of thescreen. In this sequence of operations, the method of the invention canbe implemented to obtain the centering HPOS and HPOS′, and possibly thedimensioning HSIZE and VSIZE′.

For this adjustment, the operations to be performed would then be asfollows:

-   -   display a calibrated image, for example a white image with a        perfectly centered cross,    -   launch the process of the invention wholly or in part,    -   modify the electronic and magnetic settings for the screen to        bring the cross to the center of the screen.

1. A display comprising a cathode ray tube for displaying an imagethereon; and a calculation circuit connected to said cathode ray tubefor centering and dimensioning the image being displayed by (a)measuring durations of vertical border edges of the image anditeratively modifying adjustment values for a horizontal centering ofthe image to obtain equal lateral vertical edges, (b) measuring thedurations of the vertical border edges of the image to calculateadjustment values for a horizontal dimension of the image to cause thevertical border edges to disappear, and (c) measuring durations ofhorizontal border edges of the image to calculate adjustment values of avertical dimension of the image, and to calculate adjustment values of avertical centering of the image to cause the horizontal border edges todisappear and to center the image vertically; said calculation circuitfurther checking stability of the image at a start of at least one ofthe steps (a), (b) and (c), by (a₀) measuring a pair of durations of atleast one of the vertical and horizontal border edges of the image, (a₁)subtracting two successive measurements to determine a variation in thedurations, (a₂) cornparing the variation with a first threshold, andstopping if the first threshold is exceeded or continuing if the firstthreshold is not exceeded, and (a₃) comparing the measurements with asecond threshold, and stopping if the second threshold is exceeded orcontinuing with at least one of steps (a), (b) and (c) if the secondthreshold is not exceeded.
 2. A display according to claim 1, furthercomprising a display calculator connected to said cathode ray tube forproviding display signals thereto.
 3. A display according to claim 2,wherein said display calculator comprises a memory for recording theadjustment values obtained.
 4. A display according to claim 1, whereinthe durations of the vertical border edges of the image are indicated byvariables T1_(HAv) and T2_(HAV); and wherein said calculation circuitfurther comprises performing: (a₄) checking that the measurements of thedurations T1_(HAV) and T2_(HAV) are a first measurement since a start ofsteps (a), (b) or (C), and if a response of the checking is negative,then passing on to a sub-step (a₆), and if the response of the checkingis positive, then passing on to a substep (a₅); and (a₅) comparing themeasurements for T1_(HAV) and T2_(HAV).
 5. A display according to claim4, wherein step (a₄) further comprises: setting a flag to a first logicvalue if T1_(HAV) <T2_(HAV) to indicate that the image is to bedisplaced towards the right; setting the flag to a second logic value ifT1_(HAV)<T2_(1HAV) to indicate that the image is to be displaced towardsthe left; and stopping the horizontal centering if T1_(HAV)=T2_(HAV),since the image is centered horizontally, and passing on to step (b). 6.A display according to claim 5, wherein said calculation circuit furthercomprises performing: (a₆) checking whether the flag is at the secondlogic value, and if the checking is a positive response, then passing onto a next sub-step (a₇), and if the checking is a negative response,then passing on to a sub-step (a₈); (a₇) checking whetherT1_(HAV)<T2_(HAV), and if T1_(HAV)=T2_(HAV) since the image ishorizontally centered, then stopping, and if the checking is a positiveresponse, then passing on to a sub-step (a₉), and if the checking is anegative response, then passing on to a sub-step (a₁₀); and (a₈)checking whether T1_(HAV)<T2_(HAV), and if T1_(HAV)=T2_(HAV), since theimage is horizontally centered, then passing on to step (b), and if thechecking is a positive response, then passing on to a sub-step (a₁₁),and if the checking is a negative response, then passing on to asub-step (a₁₂).
 7. A display according to claim 6, wherein saidcalculation circuit further comprises performing: (a₉) decrementing byone unit the adjustment value for centering the image to displace theimage by one incremental step to the left, and passing on to a sub-step(a₁₃); (a₁₀) incrementing by one unit the adjustment value for centeringthe image to displace the image towards the right, since the image hadbeen displaced by one incremental step too far towards the left, andstopping the horizontal centering since the image is centeredhorizontally and passing on to step (b); (a₁₁) incrementing by one unitthe adjustment value for centering the image to displace the image byone incremental step to the right, and passing on to sub-step (a₁₃);(a₁₂) decrementing by one unit the adjustment value for centering theimage to displace the image towards the left, since the image had beendisplaced by one incremental step too far towards the right, andstopping the horizontal centering since the image is centeredhorizontally and passing on to step (b); (a₁₃) incrementing a loopcounter by one unit each time the sub-step (a₉) or (a₁₁) has beenperformed, then passing on to a following sub-step (a₁₄); and (a₁₄)checking whether content of the loop counter has attained a thirdthreshold, and if the checking is a positive answer, then stopping onaccount of an impossibility of adjustment, and if the checking is anegative answer, then restarting the loop at step (a₂).
 8. A displayaccording to claim 1, wherein the durations of the vertical border edgesof the image are indicated by variables T_(HAV) and T2_(HAV); andwherein calculating the adjustment of the horizontal dimension of theimage in step (b) is based upon the formula:${HSIZE} = {\frac{{A_{Vopti}\left( {T^{\prime}/{Td}^{\;\prime}} \right)} - H_{AMPMIN}}{H_{AMPMAX} - H_{AMPMIN}} \times {HSIZE}_{MAX}}$wherein: A_(vopti) is an optimum amplitude of a current in a horizontaldeflection coil of the cathode ray tube to obtain an image of optimumwidth, with the amplitude being measured for a type of cathode ray tube,HSIZE_(MAX) is a maximum value of the adjustment of the horizontaldimension of the image, H_(AMPMAX) is a maximum variation of a currentin the horizontal deflection coil to obtain a maximum horizontaldeflection, this value varies as a function of a horizontal scanningfrequency; H_(AMPMIN) is a minimum variation of the current in thehorizontal deflection coil to obtain a minimum horizontal deflection,this value varies as a function of the horizontal scanning frequency; T′is a total duration of a period T of a horizontal synchronizationsignal, from which are subtracted a duration of a flyback pulse TFLYBACKand a duration of safety margins, andTd′=T−(T1_(HAV)+T2_(HAV)+T_(FLYBACK)), which is a duration of the imageon the screen between the border edges.
 9. A display according to claim8, wherein the coefficient (T′/Td′) in the formula is replaced by1.8T′−Td′/2.8Td′ to take into account a form of a curve for the currentin the horizontal deflection coil.
 10. A display according to claim 1,wherein the durations of the horizontal border edges of the image areindicated by variables T1_(VAV) and T2_(VAV); and wherein calculatingthe adjustment of the vertical dimension of the image in step (c) isbased upon the formula:VSIZE′=0.5[(3VSIZEMAX+2VSIZE)].[(Td×T′)/(Td′×T)]1.5 (VSIZEMAX); andwherein calculating the adjustment of the vertical centering of theimage in step (c) is based upon the formula:VPOS′=VPOS+(A−B), withA=[(2.25+1.5.(VSIZE/VSIZEMAX)]×[(0.5−T1/T′).(VPOSMAX/0.6)], and B=[(2.251.5. (VSIZE′/VSIZEMAX)]×[0.5−T1/T′). (VPOSMAX/0.6)], wherein: Td is aduration of a reference image such that:Td=T2−T1 with T1=T1_(VAV) and T2−T−T2_(VAV), T is a total duration of avertical scanning sawtooth, Td′ is a duration of the image to dimensionand to center such that:Td′=T2′−T1′ with T1′=T1′_(VAV) and T2′=T′−T2′_(VAV), VSIZE is anadjustment value for the vertical dimension of the reference image,VSIZEMAX is a maximum adjustment value for the vertical dimension of theimage, VPOS is an adjustment for the vertical centering of the referenceimage, and VPOSMAX is a maximum adjustment value for the verticalcentering.
 11. A display comprising a cathode ray tube for displaying animage thereon; and a calculation circuit connected to said cathode raytube for horizontal centering and horizontal dimensioning the imagebeing displayed on said screen by (a) measuring durations of verticalborder edges of the image and iteratively modifying adjustment valuesfor a horizontal centering of the image to obtain equal lateral verticaledges, and (b) measuring the durations of the vertical border edges ofthe image to calculate adjustment values for a horizontal dimension ofthe image to cause the vertical border edges to disappear; saidcalculation circuit further checking for stability of the image by (a₀)measuring a pair of durations of at least one of the vertical andhorizontal border edges of the image, (a₁) subtracting two successivemeasurements to determine a variation in the durations, (a₂) comparingthe variation with a first threshold, and stopping if the firstthreshold is exceeded or continuing if the first threshold is notexceeded, and (a₃) comparing the measurements with a second threshold,and stopping if the second threshold is exceeded or continuing with atleast one of steps (a) and (b) if the second threshold is not exceeded.12. A display according to claim 11, further comprising a displaycalculator connected to said cathode ray tube for providing displaysignals thereto.
 13. A display according to claim 11, wherein thedurations of the vertical border edges of the image are indicated byvariables T1_(HAV) and T2_(HAV); and wherein said calculation circuitfurther comprises performing: (a₄) checking that the measurements of thedurations T1_(HAV) and T2_(HAV) are a first measurement since a start ofsteps (a) or (b), and if a response of the checking is negative, thenpassing on to a sub-step (a₆), and if the response of the checking ispositive, then passing on to a sub-step (a₅); and (a₅) comparing themeasurements for T1_(HAV) and T2_(HAV).
 14. A display according to claim13, wherein step (a₄) further comprises: setting a flag to a first logicvalue if T1_(HAV)<T2_(HAV) to indicate that the image is to be displacedtowards the right; setting the flag to a second logic value ifT1_(HAV)<T2_(HAV) to indicate that the image is to be displaced towardsthe left; and stopping the horizontal centering if T1_(HAV)=T2_(HAV),since the image is centered horizontally, and passing on to step (b).15. A display according to claim 14, wherein said calculation circuitfurther comprises performing: (a₆) checking whether the flag is at thesecond logic value, and if the checking is a positive response, thenpassing on to a next sub-step (a₇), and if the checking is a negativeresponse, then passing on to a substep (a₈); (a₇) checking whetherT1_(HAV)<T2_(HAV), and if T1_(HAV)=T2_(HAV) since the image ishorizontally centered, then stopping, and if the checking is a positiveresponse, then passing on to a sub-step (a₉), and if the checking is anegative response, then passing on to a sub-step (a₁₀); and (a₈)checking whether T1_(HAV)<T2_(HAV), and if T1_(HAV)=T2_(HAV), since theimage is horizontally centered, then passing on to step (b), and if thechecking is a positive response, then passing on to a sub-step (a₁₁),and if the checking is a negative response, then passing on to asub-step (a₁₂).
 16. A display according to claim 15, wherein saidcalculation circuit further comprises performing: (a₉) decrementing byone unit the adjustment value for centering the image to displace theimage by one incremental step to the left, and passing on to a sub-step(a₁₃); (a₁₀) incrementing by one unit the adjustment value for centeringthe image to displace the image towards the right, since the image hadbeen displaced by one incremental step too far towards the left, andstopping the horizontal centering since the image is centeredhorizontally and passion to step (b); (a₁₁) incrementing by one unit theadjustment value for centering the image to displace the image by oneincremental step to the right, and passing on to sub-step (a₁₃); (a₁₂)decrementing by one unit the adjustment value for centering the image todisplace the image towards the left, since the image had been displacedby one incremental step too far towards the right, and stopping thehorizontal centering since the image is centered horizontally andpassing on to step Cb); (a₁₃) incrementing a ioop counter by one uniteach time the sub-step (a₉) or (a₁₁) has been performed, then passing onto a following sub-step (a₁₄); and (a₁₄) checking whether content of theloop counter has attained a third threshold, and if the checking is apositive answer, then stopping on account of an impossibility ofadjustment, and if the checking is a negative answer, then restartingthe loop at step (a₂).
 17. A display according to claim 11, wherein thedurations of the vertical border edges of the image are indicated byvariables T1_(HAV) and T2_(HAV); and wherein calculating the adjustmentof the horizontal dimension of the image in step (b) is based upon theformula:${HSIZE} = {\frac{{A_{Vopti}\left( {T^{\prime}/{Td}^{\prime}} \right)} - H_{AMPMIN}}{H_{AMPMAX} - H_{AMPMIN}} \times {HSIZE}_{MAX}}$wherein: A_(vopti) is an optimum amplitude of a current in a horizontaldeflection coil of the cathode ray tube to obtain an image of optimumwidth, with the amplitude being measured for a type of cathode ray tube,HSIZE_(MAX) is a maximum value of the adjustment of the horizontaldimension of the image, H_(AMPMAX) is a maximum variation of a currentin the horizontal deflection coil to obtain a maximum horizontaldeflection, this value varies as a function of a horizontal scanningfrequency; H_(AMPMIN) is a minimum variation of the current in thehorizontal deflection coil to obtain a minimum horizontal deflection,this value varies as a function of the horizontal scanning frequency; T′is a total duration of a period T of a horizontal synchronizationsignal, from which are subtracted a duration of a flyback pulseT_(FLYBACK) and a duration of safety margins, and Td′=T(T1_(HAV)+T2_(HAV)+T_(FLYBACK)), which is a duration of the image on thescreen between the border edges.
 18. A display according to claim 17,wherein the coefficient (T′/Td′) in the formula is replaced by1.8T′−Td′/2.8Td′ to take into account a form of a curve for the currentin the horizontal deflection coil.
 19. A display comprising a cathoderay tube for displaying an image thereon; and a calculation circuitconnected to said cathode ray tube for centering and dimensioning theimage being displayed by (a) measuring durations of vertical borderedges of the image and iteratively modifying adjustment values for ahorizontal centering of the image to obtain equal lateral verticaledges, (b) measuring the durations of the vertical border edges of theimage to calculate adjustment values for a horizontal dimension of theimage to cause the vertical border edges to disappear, and (c) measuringdurations of horizontal border edges of the image to calculateadjustment values of a vertical dimension of the image, and to calculateadjustment values of a vertical centering of the image to cause thehorizontal border edges to disappear and to center the image vertically;said calculation circuit indicates durations of the vertical borderedges of the image by variables T1_(HAV) and T2_(HAV); and calculatesthe adjustment of the horizontal dimension of the image in step (b) isbased upon the formula:${HSIZE} = {\frac{{A_{Vopti}\left( {T^{\prime}/{Td}^{\prime}} \right)} - H_{AMPMIN}}{H_{AMPMAX} - H_{AMPMIN}} \times {HSIZE}_{MAX}}$wherein: A_(vopti) is an optimum amplitude of a current in a horizontaldeflection coil of the cathode ray tube to obtain an image of optimumwidth, with the amplitude being measured for a type of cathode ray tube,HSIZE_(MAX) is a maximum value of the adjustment of the horizontaldimension of the image, H_(AMPMAX) is a maximum variation of a currentin the horizontal deflection coil to obtain a maximum horizontaldeflection, this value varies as a function of a horizontal scanningfrequency, H_(AMPMIN) is a minimum variation of the current in thehorizontal deflection coil to obtain a minimum horizontal deflection,this value varies as a function of the horizontal scanning frequency, T′is a total duration of a period T of a horizontal synchronizationsignal, from which are subtracted a duration of a flyback pulseT_(FLYBACK) and a duration of safety margins, and Td′=T(T1_(HAV)+T2_(HAV)+T_(FLYBACK)), which is a duration of the image on thescreen between the border edges.
 20. A display according to claim 19,further comprising a display calculator connected to said cathode raytube for providing display signals thereto, and wherein said displaycalculator comprises a memory for recording the adjustment valuesobtained.
 21. A display according to claim 19, wherein said calculationcircuit further checks for stability of the image by (a₀) measuring apair of durations of at least one of the vertical and horizontal borderedges of the image; (a₁) subtracting two successive measurements todetermine a variation in the durations; (a₂) comparing the variationwith a first threshold, and stopping if the first threshold is exceededor continuing if the first threshold is not exceeded; and (a₃) comparingthe measurements with a second threshold, and stopping if the secondthreshold is exceeded or continuing with at least one of steps (a) and(b) if the second threshold is not exceeded.
 22. A display according toclaim 21, wherein the durations of the vertical border edges of theimage are indicated by variables T1_(HAV) and T2_(HAV); and wherein saidcalculation circuit further comprises performing: (a₄) checking that themeasurements of the durations T1_(HAV) and T2_(HAV) are a firstmeasurement since a start of steps (a), (b) or (c), and if a response ofthe checking is negative, then passing on to a sub-step (a₆), and if theresponse of the checking is positive, then passing on to a sub-step(a₅); and (a₅) comparing the measurements for T1_(HAV) and T2_(HAV). 23.A display according to claim 22, wherein step (a₄) further comprises:setting a flag to a first logic value if T1_(HAV)<T2_(HAV) to indicatethat the image is to be displaced towards the right; setting the flag toa second logic value if T1_(HAV)<T2_(HAV) to indicate that the image isto be displaced towards the left; and stopping the horizontal centeringif T1_(HAV)=T2_(HAV), since the image is centered horizontally, andpassing on to step (b).
 24. A display according to claim 23, whereinsaid calculation circuit further comprises performing: (a₆) checkingwhether the flag is at the second logic value, and if the checking is apositive response, then passing on to a next sub-step (a₇), and if thechecking is a negative response, then passing on to a sub-step (a₈);(a₇) checking whether T1_(HAV)<T2_(HAV), and if T1_(HAV)=T2_(HAV) sincethe image is horizontally centered, then stopping, and if the checkingis a positive response, then passing on to a sub-step (a₉), and if thechecking is a negative response, then passing on to a sub-step (a₁₀);and (a₈) checking whether T1_(HAV)<T2_(HAV), and if T1_(HAV)=T2_(HAV),since the image is horizontally centered, then passing on to step (b),and if the checking is a positive response, then passing on to asub-step (a₁₁), and if the checking is a negative response, then passingon to a sub-step (a₁₂).
 25. A display according to claim 24, whereinsaid calculation circuit further comprises performing: (a₉) decrementingby one unit the adjustment value for centering the image to displace theimage by one incremental step to the left, and passing on to a sub-step(a₁₃); (a₁₀) incrementing by one unit the adjustment value for centeringthe image to displace the image towards the right, since the image hadbeen displaced by one incremental step too far towards the left, andstopping the horizontal centering since the image is centeredhorizontally and passing on to step (b); (a₁₁) incrementing by one unitthe adjustment value for centering the image to displace the image byone incremental step to the right, and passing on to sub-step (a₁₃);(a₁₂) decrementing by one unit the adjustment value for centering theimage to displace the image towards the left, since the image had beendisplaced by one incremental step too far towards the right, andstopping the horizontal centering since the image is centeredhorizontally and passing on to step (b); (a₁₃) incrementing a loopcounter by one unit each time the substep (a₉) or (a₁₁) has beenperformed, then passing on to a following sub-step (a₁₄); and (a₁₄)checking whether content of the loop counter has attained a thirdthreshold, and if the checking is a positive answer, then stopping onaccount of an impossibility of adjustment, and if the checking is anegative answer, then restarting the loop at step (a₂).
 26. A displayaccording to claim 19, wherein the coefficient (T′/Td′) in the formulais replaced by 1.8T′−Td′/2.8Td′ to take into account a form of a curvefor the current in the horizontal deflection coil.